He completed preparatory studies and graduated from the College of New Jersey (later named Princeton University) in 1836.

Hager studied law and was admitted to the bar in 1840, practicing in Morristown.

He practiced law in San Francisco and was a member of the 1849 California Constitutional Convention.

In 1865, Hager returned to the state senate, serving until 1871, when he was elected a regent of the University of California Hager was elected to the U.S. Senate in 1873 as an Anti-Monopoly Democrat, filling the vacancy caused by the resignation of Eugene Casserly.

Hager died in San Francisco on March 19, 1890, one week after his 72nd birthday, and was interred at Bellefontaine Cemetery in St. Louis, Missouri.